With regard to the shape memory alloy (hereinafter referred to sometimes as “SMA”), even if it is plastically deformed due to receiving force in a temperature not higher than martensitic transformation completion temperature, it recovers its shape when it is heated to the temperature that is not less than reverse transformation completion temperature.
FIG. 23 is a diagram wherein a relation between temperature and deformation of the shape memory alloy is graphed schematically. In FIG. 23, the horizontal axis represents temperature (° C.) and the vertical axis represents deformation (%).
As shown in FIG. 23, when electricity is supplied between both ends of the shape memory alloy at a low temperature, the shape memory alloy is contracted by generated heat to return to its memorized length. On the other hand, when the electricity supply stops at the state of this high temperature, the temperature of the shape memory alloy decreases due to heat radiation, and its length changes with hysteresis to become the state of elongated again. The shape memory alloy is possible to use as an actuator by utilizing this effect of shape memory, and there have been made various proposals.
However, actions of a shape memory alloy (SMA) are provided by heating SMA with Joule heat through supplying electricity for SMA, and thereby obtaining displacement of a driven member by utilizing a deformation corresponding to the temperature resulted from the heating. Therefore, it has been difficult to determine the unique input condition for SMA to obtain desired displacement, because of various un-uniformity in the constituted system such as, for example, errors in a length of SMA, errors in a resistance value of SMA, errors of mechanical dimensions of constituent members and the ambient temperature.
For dissolving the aforesaid problems, therefore, there has been proposed a position-control drive device that detects a position of a lens group representing a driven body, and partially changes the shape memory alloy based on the results of the detection (for example, see Patent Document 1 Japanese Patent Publication Open to Public Inspection No. 10-307628)
Further, Japanese Patent Publication Open to Public Inspection (JP-A) No. 11-324896 discloses a drive mechanism that detects the ambient temperature with a temperature sensor, and controls a current value and a voltage value to be supplied to a wire formed by a shape memory alloy, or controls a duty ratio of a pulse current or of a pulse voltage to be supplied to the wire, based on the detected result.
Further, JP-A No. 2002-99019 discloses a drive mechanism using a string-like shape memory alloy which is formed to be in a doglegged shape to be in contact with a driven body at the substantial center position of the string-like shape memory alloy and to be fixed at both ends of the string-like shape memory alloy.